1. Field of the Invention
This invention relates generally to means for clamping sheet materials, and in particular to a sheet positioning, clamping, and tensioning means for use with a xeroprinting master sheet.
2. Description of the Prior Art
An image-bearing member used to print many copies of a subject, document, or page is generally known as a master plate or sheet, or simply a "master". There are various types of masters known for use in printing or duplicating machines. A xeroprinting master is mounted on a master cylinder in a xeroprinting engine so as to be rotated past a series of process subassemblies, or stations. Ultimately a transferable image is created and transferred to a receiver to produce a print or reproduction. For simplicity, the term master as used herein refers to flexible sheets and includes printing plates and other master image-bearing substrates known in the industry.
In the production of a composite image, the transferable images generated from several masters are aligned for accurate superimposed registration during the creation of the composite. A multicolor composite print will suffer from undesirable color shift and fringing if the color separation masters are not registered, or if such registration is not accurately maintained. Even in the somewhat simpler task of providing acent color images, wherein as few as two masters are used, image registration is nonetheless critical to an acceptable composite print.
Various clamping or positioning devices are known in the art for securing both unpunched and pre-punched masters in a registered position. Those that hold unpunched masters typically rely on a gripping plate or similar mechanism to frictionally retain the master in position. Such devices are undesirable for applications which require precise registration because the master is liable to slip. These devices may use sharp edges, teeth, or prongs which puncture, tear, crease, or otherwise undesirably damage the master. Furthermore, such prior art devices typically provide accurate registration only after the master is positioned, and repositioned, by trial and error. Such repeated handling of the master is tedious and tends to deform or otherwise damage the master.
Pre-punched masters are conventionally held by two or more clamps. Pins on each clamp engage corresponding pre-punched holes. One clamp holds the leading edge of the master, and a second clamp holds the trailing edge. However, such clamps and other similar means which locate the two ends of the master at two separate areas on the master cylinder have been found to skew the master. That is, the leading and trailing ends of the master are clamped by separate devices which are not registered to each other. In general, such clamps are also bulky, complex to use, and expensive to construct.
A master is typically placed under tension to ensure a smooth and secure fit to the supporting device, which is typically a cylinder. However, prior art tensioning devices have been found to uncontrollably deform the master.
In particular, film masters formed of a polymer base, such as ESTAR.TM. film base produced by Eastman Kodak Company, are susceptible to elongation due to elastic and plastic deformation components. Elastic deformation occurs as the master is put under tension. After the tension is established, elastic deformation subsides and the material will plastic flow or "creep" as a function of time. Therefore, the tension will decrease. Some of the creep does not revert to zero when the tension is removed (i.e., the master is permanently deformed.) Unless the aforementioned effects of master deformation are accommodated, accurate registration of the master is difficult if not impossible to achieve.
A further problem arises in the prior art because the strain of the master material is not made uniform over the length of the master. A non-uniformity, or rippling, of the master at its longitudinal edges is experienced, and accurate registration of the master is frustrated.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.